The invention relates to a method for the tool-free reshaping of a tubular body of amorphous, especially vitreous material, to a solid rod by pressure constriction, for example, vacuum constriction or constriction due to thermal treatment, wherein a depression of pressure below atmospheric pressure is maintained in the tubular piece by continual evacuation as it closes on itself, while the hollow body is fed horizontally, in a continuous manner, with a given rotational speed, to a heating zone where its viscosity is so lowered that, due to the pressure difference between the absolute value of pressure below atmospheric pressure in the hollow body and the external pressure acting on the body, the body is constricted within a closing zone to form the rod, and the rod is continuously withdrawn from the closing zone at a given rotational speed.
Such processes are generally known and are used chiefly for constricting hollow cylinders of doped quartz glass to produce solid cylindrical preforms for fiber optic technology. The hollow cylinders are produced, for example, by the flame-hydrolytic deposition of glass particles on a mandrel, which can consist, for example, of glass, graphite or aluminum oxide, resulting in an open-pored "soot body" which is then thermally compressed. The mandrel is removed, for example, by drilling it out, etching it out, or drawing it out. To achieve a smooth and uniform inside surface, the inside of the hollow cylinder is generally smoothed or polished. To dry and clean the inner surfaces of the cylinder these are frequently treated with gases containing fluorine or chlorine, as disclosed for example in French examined and published patent application 2,441,594, to remove volatile halides and thereby etch away the surfaces. On account of their chemical reactivity these cleaning and drying gases, however, attack all other surfaces, e.g., those of the furnace, and they are extremely toxic.
It is known from German Patent 28 27 303 that the constricting process is facilitated and accelerated by applying a vacuum in the hollow cylinder. As the tube walls close up in the closing zone under a vacuum they move more rapidly toward one another due to forces produced by the vacuum which act radially inwardly, so that random asymmetries in the geometry of the tube can result in premature, irreversible contact between confronting walls, leading to flattening and distortion in the core area of the rod. For example, it is stated in "Polarization characteristic of non-circular core single-mode fibers," by V. Ramaswamy, W. G. French and R. D. Standley, Applied Optics, Vol. 17 No. 18, 1978, pages 3014 to 3017, that a high vacuum causes a dumbbell-shaped deformation and a low vacuum causes an oval deformation of the core region. In European Patent 0 032 390 there is described the production of a polarizing fiber optic with an oval cross section in the core region. To obtain the oval shape in the core region, vacuums of about -0.1 mbar to -2 mbar below the external pressure acting on the cylinder are maintained during the constriction of hollow cylinders of doped quartz glass with outside diameters of about 20 mm and inside diameters of about 17 mm. In the process described, the entire inside surface of the hollow cylinder is replicated at the center of the solid cylinder after constriction. Inevitable centers of distribution on the inside surface of the hollow cylinder due to impurities, moisture, surface defects, or to the fact that a surface layer always has a different stoichiometry than the solid material, lead to inhomogeneities in the center of the rod, where they are particularly undesirable in general.
An object of the present invention is therefore to devise a process for making uniform, rod-shaped bodies from hollow bodies in a single operation, at low cost and in a tool-free manner.